Amplifier arrangement



Patented Dec. 31 1933 UNITED STATES PATENT OFFICE AMPLIFIER ARRANGEMENT tion of Germany Application October 18, 1932, Serial No. 638,300

, In Germany December 29, 1931 9 Claims.

An object of the invention is to provide a novel circuit arrangement for an amplifier.

A second object is to provide a resistance coupling to connect the output circuit of an amplifier 5 with an external load and adjustable means connected to the input electrodes of the amplifier whereby it will be caused to operate on a linear characteristic curve.

A still further object is to provide separate 10 means for varying the grid bias of an amplifier and the feedback of energy between its output and input circuits. Other objects of the invention will appear as the description thereof proceeds. For the purpose of explanation the invention is shown in the accompanying drawing, in which,

Fig. 1 is a schematic diagram showing one form .of the invention,

Fig. 2 is a schematic circuit arrangement show- 320 ing a second embodiment of the invention,

Fig. 3 is a schematic circuit arrangement showing a still further embodiment of the invention, .and,

Fig. 4 shows a circuit arrangement similar to 25 Fig. 2 with the addition of .a bias battery. In the several figures the same reference numerals are used to denote similar or identical electrical elements.

I have found that the distortion produced in 30 an audion or vacuum tube'amplifier, whether due to operating on a portion of the characteristic curve of the amplifier which is curved or nonlinear or to a variable resistance or reactance in the output circuit which varies in a non-linear 5 manner, may be substantially reduced or prevented by means of a counter-coupling or feedback action between the output and input circuits. This reversed feedback may slightly reduce the total gain or amplification but this loss is more- 40 than compensated for by the fidelity of the amplification. This is especially true with respect to the transition from an amplifier stage operating without energy into a power amplification stage inasmuch as linear voltage amplification practi- 45 cally free from energy expenditure is now available.

Referring to Fig. 1 the vacuum tube amplifier having plate I, control grid 2 and cathode 3 has a grid or input circuit including the primary 4 50 and secondary 5. As drawn, an iron core 6 for the input transformer is shown but this is not essential if my invention is to be used to amplify radio frequency currents. A high resistance 1 is connected to the plate and filament by leads 8 55 and 9 and a contact I adjustable along resistor I to any desired position is connected to one end of secondary 5, the opposite end of which is connected to the grid 2. A battery ll of suitable voltage to supply the necessary plate current is connected in lead 8. A bias or 0 battery l2 may be inserted in the grid circuit to assist in biasing the grid negative with respect to the filament. The input terminals I3 are connected to primary 4 and the output terminals l4 may be connected to the ends of resistor 1.. It will be noted that 10 the lower end of l is at cathode potential while its upper end is below the cathode potential due to the direct plate current fiow through "I. By a proper choice of the value of resistor l and battery II the grid bias may be given any desired value by means of sliding contact I0, with the result that part of the alternating potential of the plate circuit is impressed on the grid under conditions of phase opposition. By suitably choosing thepoint of application of ID, the opposite coupling and the requisite negative grid biasing potential may be tapped directly from theexternal resistance of the tube without the provision of a separate C battery I2, such tapping being effected-not only in the portion of the plate circuit carrying direct current, but also in the portion carrying the alternating current component. The position of 7 contact 10 also governs the value of the countercoupling or compensating fluctuating potential applied to the grid from the plate circuit. By a trol voltage applied tothe grid-2 to secure this result is obtained by properly positioning contact It on resistor or impedance 1.

Figure 2 shows an arrangementgenerallysimi lar to that of Fig. 1 and in which the compensating or counter-coupling voltage is applied to the grid through a capacity C whose impedance, inside the required frequency range, is of small value compared with the impedance Z of the direct current supply which may be a battery similar to l2 or other desired source of voltage. By variation of the contact l5 it is thus feasible to insure a regulation of the gain of the amplifier, since with growth of counter-coupling the ensuing gain is diminished. In other words the initial bias given to the grid to bring the operating point on a linear portion of the grid voltageplate current characteristic curve may be secured by adjustment of contact ID, the contact I 5 then being adjusted to cause such compensating voltage to be applied to the grid as to main- 5 5 tain linear operation of the tube over a desired frequency range or band.

As shown in Fig. 3 the plate resistance is disposed in two parallel branches '1' and 1", contact Ill being adjustable along one branch and contact I5 of the condenser along the other. By this means adjustment of the initial grid bias by contact 10 is made independent of the compensating voltage supplied through contact I5 and condenser C.

If the direct current biasing potential is derived partly from the plate resistance and partly from a battery or other fixed source, as shown in Figs. 1 and 4, the circuit constants may be so chosen that the feed current, i. e. the fall of D. 0. potential in the external resistance remains substantially constant regardless of variations in the plate potential. Also, in the circuit arrangement shown in Fig. 3, if resistor 1' has a value for example two or more times larger than that of 1", the D. 0. grid bias will remain substantially constant even with relatively large variations of plate potential due to applied signal voltages on the grid, as the alternating component of the plate current will flow mostly through branch 1.

What I claim as my invention is:

1. Means for amplifying audio frequency currents comprising the combination of a vacuum tube having a cathode, grid and plate, a source of plate current having its positive terminal connected to said plate, a high resistance having one endconnected to the negative terminal of said source, a lead directly connecting the other end of said resistor and said cathode, an input circuit including an inductive reactance having one end connected to said grid and a source of constant voltage having its negative terminal directly connected to the other end of said inductive reactance and a ,lead only connecting its positive terminal to an intermediate point of said high resistance, said constant voltage source having such value as to bias the grid negative by such amount as to initiate operation of said amplifier on the linear portion of its characteristic curve.

2. A vacuum tube having a plate, grid and cathode, a plate circuit connecting said plate and cathode and including a voltage source and a resistor, an input circuit connected to said grid and cathode, means for including any desired portion of said resistor in said input or grid circuit and a condenser having one side connected to said input circuit and its opposite side connected to an intermediate point on said resistor.

3. In an amplifier, the combination of a vacuum tube having a cathode, grid and plate, an impedance and a source of direct current connected in series between said cathode and plate, an input circuit connected to said cathode and grid including a tap connection to an intermediate point on said impedance whereby said grid voltage lies on a linear portion of the grid voltage-plate current characteristic curve and means for maintaining the linearity of said portion 5 over a range of grid voltages comprising a capacitor having one plate connected to said input circuit and a second plate connected to a second intermediate point on said impedance.

4. In combination, a vacuum tube amplifier 10 'having input and output circuits, an impedance in said output circuit, one end of said input circuit having separate branches, and separate adjustable means for simultaneously connecting said branches to difierent points on said im- 5 pedance.

5. A vacuum tube having a plate, grid and cathode, an impedance and a voltage source connecting said cathode and plate, means connecting said grid and cathode including a portion of 20 said impedance having such value as to give said grid such bias as to initiate operation of said tube on the linear portion of its characteristic curve and a second connection between said grid and said impedance arranged to impose voltage 25 fluctuations on said grid of such value as to maintain operation of said tube on the linear portion of its characteristic as a fluctuating voltage is imposed on the grid circuit.

6. In combination, a vacuum tube having a 30 cathode, grid and plate, an impedance and a source of voltage connecting said cathode and plate, said impedance having two branches connected in parallel, a circuit connecting said grid to an intermediate point of one branch and 36 means connecting said grid and an intermediate point of the other branch.

7. The combination of the preceding claim in which said last named means includes a capacitor. 40

8. In combination, a vacuum tube having a cathode, grid and plate, an impedance having two branches connected between said cathode and plate, a circuit connecting said grid and cathode and including means for connecting a 15 portion of one branch in both said grid and plate circuits and a circuit connecting the other branch of said impedance with said grid.

9. In combination, a vacuum tube having grid and plate circuits, an impedance at least a por- 50 tion of which is common to said named circuits whereby said grid is given a desired negative bias and additional means for applying voltage fluctuations from a portion of said impedance different than said first named portion to said grid, 55 said voltage fluctuations being of sufiicient amplitude to cause said tube to operate as a substantially linear amplifier.

WERNER BUSCHIBECK. 

